The present invention relates to gas lasers. The present invention also relates to dedusting units for the laser optical elements of gas lasers and to methods for assembling dedusting units for laser optical elements in gas lasers.
Lasers have recently been applied to a large variety of technical areas, such as optical measurement techniques, material processing, medicine, etc.
Thus, there is a big demand on providing near diffraction limited lasers, i.e. lasers generating laser beams having a short wavelength.
The excimer laser, such as the one disclosed in U.S. Pat. No. 5,771,258, serves well as a laser for generating coherent light.
The excimer laser described in U.S. Pat. No. 5,771,258 is a pulsed laser. Pulsing a is required in excimer lasers to allow sufficient time between pulses to replace the laser gas within the discharge region with fresh gas and allow the gas used for generating the previous pulse to recover before being used again for another gas discharge. In the discharge region (i.e., discharge gap), which in an excimer laser is typically defined between an elongated high voltage electrode and an elongated ground electrode which are spaced apart from each other, a pulsed high voltage occurs, thereby initializing emissions of photons which form the laser beam.
The laser beam is emitted along the extended ground electrode in a longitudinal direction of the laser tube. The laser beam leaves the tube through a window of the tube.
Unfortunately dust within the laser tube deteriorates the optical performance of the optical elements within the tube, for example the windows and/or mirrors in the tube that define the laser resonator.
There have been made several efforts to solve this problem.
For example, Japanese Patent No. 6 237 034 discloses a discharge excited gas laser device with negative electrodes having holes, through which the laser beam can pass. The negative electrodes are mounted to the optical window sections in the laser tube. Negative voltage is applied to the electrodes by a negative power supply to thereby charge the surface of the optical windows, which would normally be brought into contact with dust in the laser tube, with a negative charge. As a result, negatively charged dust particles in the tube are prevented from adhering to the optical windows.
Furthermore, Japanese Patent No. 5 067 823 discloses a discharge excitation pulse gas laser apparatus with a static gas chamber provided near an optical window at the end section of the laser tube. The static gas chamber includes a space that is designed to ensure no gas flow occurs within the chamber due to the gas circulation in the rest of the laser, thus making it hard for the metallic dust particles to reach the surface of the window even if the gas is agitated by a gas circulation means, such as a fan, in the laser tube.
German Patent Application DE 198 400 35 discloses an excimer laser with a pair of electrodes in the region of the windows. A high voltage is applied to the electrodes to control a plasma gas flow through the output window. The electrodes located in the region of the windows thus generate a directed gas flow. Further, a series of slots in the electrodes are electrostatically charged to attract dust particles. In the laser tube, a main electrode arrangement is provided for creating a gas discharge. In addition, a gas cleaning unit is provided.
The laser tubes according to the state of the art as mentioned above use a technology, in which the optical system itself or the limiter of the reduced flow area is charged. This leads to a large surface to be charged and thus, high-energy losses occur with only low field gradients of the electrical field being generated. The charged elements push off precharged metallic dust particles. Another disadvantage of the devices described above is their rather complex construction, which makes them expensive and susceptible to malfunctions.
The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application:
All of the foregoing applications are incorporated by reference as if fully set forth herein.
A first object of the invention is to provide a dedusting unit for a laser optical element with improved properties, especially with reference to the complexity of construction.
To achieve the first object, a dedusting unit for a laser optical element is provided, which comprises a high-voltage duct comprising a high-voltage conducting core having a first end and a second end and an insulator element disposed around the core, the first end of the core being connectable to a high voltage power supply, and a wire loop electrically connected to the second end of the high-voltage core.
The optical element to be protected by the dedusting unit may be any of the optical elements used in a gas laser. As a result, the optical element to be protected may be a fully reflective mirror, a partially transparent, partially reflective mirror, or a fully transparent window.
Thus, the present invention provides a very simple device for preventing dust from reaching an optical element of the laser tube, especially the output window, through which the laser light is emitted, or the 100% reflecting mirror providing resonant laser light. Because the construction is very simple and small, the dedusting unit according to the invention may be readily mounted in front of the mirrors and windows in the tube.
Furthermore, the dedusting unit according to the invention may easily be detached from its mounting. A further advantage of the device is that an electrical field with very high field gradients is generated by the wire loop when the dedusting unit is connected to a high voltage power supply, thereby significantly improving the dedusting effect compared with the devices according to the state of the art.
The wire loop may have any loop form. Preferably, however, the wire loop has a substantially circular form having a diameter that is sufficient to allow the resonating laser light within the tube to pass through the wire loop without being obstructed.
A second object of the invention is to provide a gas laser comprising a laser optical element with an improved dedusting unit, especially with reference to the complexity of the construction of the dedusting unit.
To achieve the second object of the invention, a gas laser is provided comprising a tube having a first end wall at one end and a second end wall at the other end and defining a cavity for containing a laser gas. An elongated high voltage electrode is disposed within the tube and extends parallel to the longitudinal axis of the tube. An elongated ground electrode is also disposed within the tube. The ground electrode extends parallel to the high voltage electrode and is spaced apart from the high voltage electrode to thereby define a gas discharge gap therebetween. A laser resonating path is also provided in axial alignment with the gas discharge gap. A first laser optical element is disposed in the laser resonating path and has a side exposed to the cavity formed by the tube. And a dedusting unit is mounted to the laser tube. The dedusting unit comprises (1) a high-voltage duct comprising a high-voltage conducting core having a first end and a second end and an insulator element disposed around the core, the first end of the core being connectable to a high voltage power supply, and (2) a wire loop electrically connected to the second end of the high-voltage core. The dedusting unit is mounted to the laser tube so that the wire loop is disposed inside the tube in proximity to the first side of the optical element, and the wire loop is transverse to the resonating path so that the resonating path passes through the wire loop.
The gas laser comprising the dedusting unit according to the present invention has improved properties with reference to maintenance and lifetime, because the optical elements, such as a window or a mirror within the tube, dirty significantly less compared with those according to the state of the art.
A third object of the invention is to provide a simple method for assembling a dedusting unit for a laser optical element in a gas laser.
To achieve this third object, a method for installing a dedusting unit for a laser optical element of a gas laser is provided, wherein the gas laser comprises a tube having a first end wall at one end and a second end wall at the other end and defining a cavity for containing a laser gas, a laser resonating path substantially parallel to the longitudinal axis of the tube and along which coherent light can resonate, and a laser optical element having a first side exposed to the cavity formed by the tube, the laser optical element being mounted to the first end wall so that the first side of the optical element is disposed in the laser resonating path, and wherein the dedusting unit for the optical element comprises (1) a high-voltage duct comprising a high-voltage conducting core having a first end and a second end and an insulator element disposed around the core, the first end of the core being connectable to a high voltage power supply, and (2) a wire loop electrically connected to the second end of the high-voltage core. A preferred method according to this third object of the invention comprises the steps of (1) flattening the wire loop into an elongated shape so that the width of the wire loop is smaller than the diameter of a bore hole extending through the first end wall, (2) inserting the wire loop through the bore until the elongated wire loop is inside the tube, (3) expanding the elongated wire loop to a desired form which is transverse to the resonating path, and (4) positioning the wire loop of desired form so that it is in proximity to the first side of the optical element and the laser resonating path passes through the wire loop.
Thus, a very simple and thus cheap method is provided for installing a dedusting unit for a laser optical element in a gas laser according to the present invention.
In accordance with a preferred embodiment of the present invention, the high-voltage duct is a waveguide like coaxial duct.
According to a further embodiment of the invention, the insulator element of the high-voltage duct comprises a cylindrical ceramic tube and the core is coaxial disposed within the ceramic tube. With this embodiment, an effective and cheap construction of the high voltage duct is provided.
The dedusting unit for the laser optical element may be installed through a bore hole that extends radially through one of the end walls of the laser in proximity to the optical element to be protected. Alternatively, the dedusting unit may extend through a bore hole that extends transverse through the end wall on which the optical element to be protected is mounted.
The dedusting unit of the present invention can be used in connection with a variety of gas lasers. It is particularly well suited to be used in connection with excimer lasers.
Preferred embodiments of the invention will now be described in detail in connection with the accompanying drawings.